Methods for forming a spacer stack for magnetic tunnel junctions

ABSTRACT

An exemplary method that forms spacer stacks with metallic compound layers is disclosed. The method includes forming magnetic tunnel junction (MTJ) structures on an interconnect layer and depositing a first spacer layer over the MTJ structures and the interconnect layer. The method also includes disposing a second spacer layer—which includes a metallic compound—over the first spacer material, the MTJ structures, and the interconnect layer so that the second spacer layer is thinner than the first spacer layer. The method further includes depositing a third spacer layer over the second spacer layer and between the MTJ structures. The third spacer is thicker than the second spacer.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.16/129,088, filed on Sep. 12, 2018 and titled “Spacer Stack for MagneticTunnel Junctions,” which claims the benefit of U.S. Provisional PatentApplication No. 62/690,724, filed on Jun. 27, 2018 and titled “SpacerStack for Magnetic Tunnel Junctions.” The entire contents of theabove-noted applications are incorporated by reference herein in theirentireties.

BACKGROUND

Magnetic tunneling junctions (MTJs) are an integral part of magneticrandom access memories (MRAMs). A fabrication process of an MTJstructure can involve a variety of operations, such as metal anddielectric depositions, photolithography, etch processes, etc. MTJs canbe formed between back end of the line (BEOL) interconnect layers andseparated bilaterally by spacer stacks which can be formed on thesidewalls of each MTJ structure.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the common practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a flowchart of an exemplary fabrication method for formingmagnetic tunnel junction structures with spacer stacks featuringmetallic compound layers, according to some embodiments.

FIG. 2 is a cross-sectional view of exemplary magnetic tunnel junctionlayers over an interconnect layer, according to some embodiments.

FIG. 3 is a cross-sectional view of magnetic tunnel junction structuresover vias in an interconnect layer after photolithography and etchoperations, according to some embodiments.

FIG. 4 is a cross-sectional view of magnetic tunnel junction structuresover an interconnect layer after the deposition of a first spacer layer,according to some embodiments.

FIG. 5 is a cross-sectional view of magnetic tunnel junction structuresover an interconnect layer after an etchback process of a first spacerlayer, according to some embodiments.

FIG. 6 is a cross-sectional view of magnetic tunnel junction structuresover an interconnect layer after a deposition of a metallic compoundlayer, according to some embodiments.

FIG. 7 is a cross-sectional view of magnetic tunnel junction structuresover an interconnect layer after an etchback process of a metalliccompound layer, according to some embodiments.

FIG. 8 is a cross-sectional view of magnetic tunnel junction structuresover an interconnect layer after deposition of a third spacer layer anda dielectric layer, according to some embodiments.

FIG. 9 is a cross-sectional view of magnetic tunnel junction structureswith a spacer stack over an interconnect layer after a chemicalmechanical planarization process, according to some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over a second feature in the description that followsmay include embodiments in which the first and second features areformed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper,” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

The term “about” as used herein indicates the value of a given quantitythat can vary based on a particular technology node associated with thesubject semiconductor device. Based on the particular technology node,the term “about” can indicate a value of a given quantity that varieswithin, for example, 10-30% of the value (e.g., ±10%, ±20%, or ±30% ofthe value).

The term “nominal” as used herein refers to a desired, or target, valueof a characteristic or parameter for a component or a process operation,set during the design phase of a product or a process, together with arange of values above and/or below the desired value. The range ofvalues can be due to slight variations in manufacturing processes ortolerances. Unless defined otherwise, technical and scientific termsused herein have the same meanings as commonly understood by one ofordinary skill in the art to which this disclosure belongs.

As used herein, the term “substantially” indicates that the value of agiven quantity varies by ±1% to ±5% of the value.

Magnetic tunneling junctions (MTJs) are an integral part of magneticrandom access memories (MRAMs). A fabrication process of an MTJstructure can involve a variety of process operations, including metaland dielectric depositions, photolithography, etch operations, etc. MTJstacks can be interposed between back end of the line (BEOL)interconnect layers and separated bilaterally by spacer stacks formed onthe sidewalls of each MTJ structure. The role of the spacer stacks is toelectrically isolate the MTJ stacks from one another. As the MRAM cellsare scaled down from one technology generation (node) to the next, thepitch (e.g., the spacing) between adjacent MTJ structures is reduced.Consequently, thinner spacer stacks are required to fill the spacebetween the MTJ structures. Keeping the MTJ structures in MRAM cellselectrically isolated when the spacing between MTJ structures is reduced(e.g., below 92 nm) can be challenging for thinner spacer stacks, suchas spacer stacks having a thickness below about 30 nm.

The embodiments described herein are directed to an exemplaryfabrication method that describes the formation of spacer stacks. Insome embodiments, the spacer stack thickness is below about 30 nm (e.g.,about 25 nm) between MTJ structures, and the spacer stack pitch isbetween about 80 nm and about 92 nm. In some embodiments, the spacerstacks can include a metallic compound layer with improved dielectricproperties (e.g., electrical isolation characteristics). The metalliccompound layers can include aluminum oxide, aluminum nitride, titaniumoxide, titanium nitride, ruthenium oxide, or any other suitable materialand can have a thickness below about 5 nm (e.g., about 3 nm). Themetallic compound layer can be interposed between layers of siliconnitride, silicon carbon nitride, silicon oxy-nitride, silicon oxy-carbonnitride, silicon oxide or combinations thereof to form a spacer stack oneach sidewall surface of the MTJ structure. In some embodiments, themetallic compound layer is deposited with thermal atomic layerdeposition or plasma-assisted atomic layer deposition. According to someembodiments, spacer stacks with metallic compound layers exhibitimproved electrical isolation characteristics compared to spacer stackswithout metallic compound layers. Consequently, spacer stack withmetallic compound layer can be more compact (e.g., thinner).

FIG. 1 is a flowchart of an exemplary fabrication method 100 thatdescribes the formation of MTJ spacer stacks with a metallic compoundlayer, according to some embodiments. According to some embodiments, themetallic compound layer can be deposited with thermal atomic layerdeposition (ALD) or plasma-enhanced atomic layer deposition (PEALD) at atemperature range between about 150° C. and about 400° C. Fabricationmethod 100 is not limited to the operations described below. Otherfabrication operations (e.g., wet cleans, additional photolithographyand deposition operations, etc.) may be performed between the variousoperations of fabrication method 100 and may be omitted merely forclarity. These fabrication operations, even though not described, arewithin the spirit and the scope of the present disclosure.

In referring to FIG. 1 , exemplary fabrication method 100 begins withoperation 110 and the formation of one or more MTJ structures over aninterconnect layer. The formation of one or more MTJ structures will bedescribed using FIGS. 2 and 3 as examples. FIG. 2 is a cross-sectionalview of blanket-deposited MTJ layers 200 over one or more interconnectlayers 205. MTJ layers 200 can include a bottom electrode 210, an MTJstack 215, and a top electrode 220. By way of example and notlimitation, MTJ stack 215 can be a multilayer structure that includes anon-conductive layer interposed between two ferromagnetic layers. Thenon-conductive layer and the ferromagnetic layers in MTJ stack 215 arenot shown in FIG. 2 for simplicity. By way of example and notlimitation, the non-conductive layer of MTJ stack 215 can be includemagnesium oxide (MgO), aluminum oxide (AlO_(x)), aluminum oxynitride(AlON), any other suitable material, or combinations thereof. Further,the non-conductive layer can be deposited by physical vapor deposition(PVD). Alternatively, the non-conductive layer may be deposited by otherdeposition methods, such as plasma-enhanced PVD (PEVD), chemical vapordeposition (CVD), plasma-enhanced CVD (PECVD), ALD, PEALD, or any othersuitable deposition method. The ferromagnetic layers of MTJ stack 215can include metal stacks with one or more layers including iron (Fe),cobalt (Co), ruthenium (Ru), magnesium (Mg), any other suitablematerial, or combinations thereof. The ferromagnetic layers may bedeposited by PVD, PEVD, CVD, PECVD, ALD, PEALD, or any other suitabledeposition method. In some embodiments, the combined thickness of MTJlayers 200 can range from about 100 Å to about 400 Å.

Top electrode 220 and bottom electrode 210 are in contact with therespective ferromagnetic layers of MTJ stack 215. By way of example andnot limitation, top electrode 220 can include tantalum (Ta), tantalumnitride (TaN), titanium nitride (TiN), tungsten (W), any other suitablematerial, or a combination thereof. Further, top electrode 220 can bedeposited by CVD or PVD methods. In some embodiments, top electrode 220can be a stack that includes TiN and TaN layers. Bottom electrode 210can include TiN, TaN, Ru, copper (Cu), any other suitable material, or acombination thereof. Bottom electrode 210 can also be deposited by CVDor PVD methods. In some embodiments, top and bottom electrodes 220 and210 can each have a thickness between about 300 Å and about 800 Å.

In some embodiments, interconnect layers 205 can be formed prior to theformation of MTJ layers 200. According to some embodiments, interconnectlayers 205 can be formed over previously formed interconnect layers,which are not shown in FIG. 2 for simplicity. These previously formedinterconnect layers may include, for example, BEOL interconnect layers,middle of the line (MOL) layers (e.g., contact interconnects), and fieldeffect transistors (FETs) formed over a substrate (e.g., a wafer). Byway of example and not limitation, interconnect layers 205 can be BEOLlayers with several vertical interconnect access lines and lateral lines(also referred to herein as “lines”). Interconnect layers 205 in FIG. 2are shown with vias 225, while lines are not included for simplicity.For example purposes, interconnect layers 205 will be described withvias 225. However, based on the disclosure herein, lines are also partof interconnect layers 205 and are within the spirit and scope of thisdisclosure. Further, the number of vias 225 shown in FIG. 2 is notlimiting and additional vias 225 are possible.

According to some embodiments, vias 225 (and the lines) of interconnectlayers 205 can be filled with a metal stack that includes at least abarrier layer 230 and a metal fill 235. Barrier layer 230 can be singlelayer or a stack of two or more layers. In some embodiments, metal fill235 can be an electroplated metal or a metal alloy. By way of exampleand not limitation, barrier layer 230 can be a tantalum (TaN)/tantalum(Ta) stack or a cobalt (Co) single layer deposited by PVD. Metal fill235 can be electroplated copper or an electroplated copper alloy, suchas copper-manganese, copper-ruthenium, or any other suitable material.The vias (e.g., vias 225) and the lines of interconnect layers 205 areembedded in interlayer dielectric (ILD) layers 240 and 245. In someembodiments, ILD layers 240 and 245 can be silicon oxide or a low-kmaterial with dielectric constant lower than the dielectric constant ofthermally grown silicon oxide (e.g., below 3.9). In some embodiments,ILD layers 240 and 245 can be a stack of dielectrics such as, forexample, a low-k dielectric and another dielectric: (i) a low-kdielectric (e.g., carbon doped silicon oxide) and a silicon carbide withnitrogen doping; (ii) a low-k dielectric and a silicon carbide withoxygen doping; (iii) a low-k dielectric with silicon nitride; or (iv) alow-k dielectric with silicon oxide. Further, ILD layers 240 and 245 canbe deposited with a high-density plasma CVD (HDPCVD) or a PECVD process.In some embodiments, ILD layer 240 can be a different from ILD layer245. For example, ILD layer 240 can be carbon doped silicon oxide (SiOC)and ILD layer 245 can be silicon oxide. In some embodiments, etch stoplayers, 250 and 255, are interposed between ILD layers 240 and 245. Byway of example and not limitation, etch stop layer 250 can includesilicon carbon nitride (SiCN) or aluminum nitride (AlN) and can have athickness between about 10 Å and about 150 Å. Etch stop layer 245 caninclude aluminum oxide and can have a thickness between about 10 Å andabout 40 Å. Etch stop layers 250 and 255 are used during the formationprocess of vias 225.

Lithography and etch operations can be used to pattern MTJ layers 200.As a result, one or more MTJ structures can be formed according tooperation 110 of method 100. For example, a mask layer (not shown inFIG. 2 ), can be disposed and patterned over top electrode 220. The masklayer may include one or more layers and have a total thickness of about750 Å. By way of example and not limitation, the mask layer can includea layer stack with a bottom oxide layer and a top amorphous carbonlayer. Any portions of MTJ layers 200 that are not covered by thepatterned mask layer can be removed during a subsequent etch operation.

As shown in FIG. 3 , MTJ structures 300 can be formed from the un-etchedportions of MTJ layers 200 over interconnect layers 205. In someembodiments, the patterned mask layer is aligned to vias 225 ofinterconnect layers 205 so that MTJ structures 300 are formed on top ofvias 225 as shown in FIG. 3 . Therefore, each bottom electrode 210 ofMTJ structures 300 can be in electrical and physical contact with arespective underlying via 225 of interconnect layer 205. Additionally,and as a result of the aforementioned etch process, the top surface ofILD layer 245 may be recessed with respect to a top surface of vias 225during the formation of MTJ structures 300. After the etch process, thepatterned mask layer on top of MTJ structures 300, which is not shown inFIG. 3 , can be removed with a wet clean process.

According to some embodiments, a pitch P between adjacent MTJ structures300 can range from about 92 nm to about 80 nm (e.g., about 82 nm)depending on the MRAM layout design. This means that the space betweenthe sidewalls of adjacent (e.g., neighboring) MTJ structures 300 can beless than about 80 nm.

Referring to FIG. 1 , method 100 continues with operation 120 and theformation of a first spacer on each sidewall of the one or more MTJstructures 300. By way of example and not limitation, the first spacerformation process can be described using FIGS. 4 and 5 . In referring toFIG. 4 , a first spacer material 400 can be blanket-deposited over MTJstructures 300 and ILD layer 245 at a thickness between about 30 Å andabout 200 Å (e.g., about 150 Å). In some embodiments, spacer material400 can include silicon nitride (SiN), silicon carbon nitride (SiCN),any suitable material, or combinations thereof. For example, firstspacer material 400 can be a single layer or a stack with a SiN bottomlayer and a SiCN top layer. In some embodiments, spacer material 400 canbe conformally deposited with an atomic layer deposition (ALD) processor a chemical vapor deposition (CVD) process at a temperature betweenabout 100° C. and about 400° C. During deposition, the process pressurecan be between about 0.5 Torr and about 10 Torr. The aforementionedprocess pressure range is exemplary and other ranges may be used.

After the deposition of first spacer material 400, an anisotropicetchback process can be used to selectively remove first spacer material400 from the horizontal surfaces of MTJ structures 300 (e.g., from topsurface of top electrode 220) and ILD layer 245. The anisotropicetchback process (e.g., a directional etch process) can be configured toexhibit higher removal rates for first spacer material 400 on horizontalsurfaces (e.g., at least 2 times higher or more) than on non-horizontalsurfaces, such as the sidewall surfaces of MTJ structures 300.Consequently, the un-etched portions of first spacer material 400 thatcover the sidewall surfaces of MTJ structure 300 can form first spacers500 as shown in FIG. 5 . In some embodiments, due to the anisotropicnature of the etchback process, first spacer material 400 is recessedfrom the top corners of top electrodes 220. Consequently, first spacers500 do not cover the entire sidewall surface of top electrode 220, asshown in FIG. 5 .

According to some embodiments, the anisotropic etchback process caninclude an ion beam etch process or a reactive ion etching (RIE) processwith an inductively coupled plasma (RIE-ICP). In some embodiments, theion beam etch process can use an ion beam that selectively targets areaswhere the etching will be performed. The ion beam can be produced from ahelium (He), neon (Ne), argon (Ar), krypton (Kr), or xenon (Xe) plasma.By way of example and not limitation, the energy of the ion beam duringthe etch process can be between about 100 eV and about 1200 eV.Alternatively, the RIE-ICP process can use etchants such astetrafluoromethane (CF₄), fluoroform (CH₂F₂), chlorine (Cl₂), Ar, He,organic gases, or combinations thereof.

In referring to FIG. 1 , method 100 continues with operation 130, wherea metallic compound layer is deposited over the first spacers, the oneor more MTJ structures 300, and top interconnect layer 205. For example,as shown in FIG. 6 , a metallic compound layer 600 can be conformallydeposited over first spacer 500, MTJ structures 300, and interconnectlayer 205. According to some embodiments, metallic compound layer 600can be conformally deposited by a thermal ALD or a plasma-enhanced ALDprocess at a thickness between about 5 Å and about 50 Å (e.g., about 30Å). In other words, the as-deposited metallic compound layer 600 can bethinner than first spacer 500. In a thermal ALD process the depositiontemperature can range from about 150° C. to about 400° C. On the otherhand, the deposition temperature for a plasma-enhanced process can belower. For example, the deposition temperature for a plasma-enhancedprocess can be between about 50° C. and about 350° C. (e.g., 100° C.,150° C., 200° C., 350° C., etc.). In some embodiments, the processpressure for both thermal and plasma-enhanced ALD can range from about0.5 Torr to about 10 Torr. The aforementioned process pressure range isexemplary and other ranges may be used. According to some embodiments,metallic compound layer 600 can include aluminum nitride (AlN_(x)),aluminum oxide (AlO_(x)), titanium nitride (TiN), titanium oxide(TiO_(x)), ruthenium oxide (RuO_(x)), or any other suitable material.For example purposes metallic compound layer 600 in method 100 will bedescribed in the context of an aluminum-based layer, such as aluminumnitride and aluminum oxide. Based on the disclosure herein, othermetallic compound layers, as discussed above, can be used. These othermetallic compound layers are within the spirit and scope of thisdisclosure.

For metallic compound layer 600 that includes aluminum nitride oraluminum oxide, a trimethylaluminum (TMA) (Al₂(CH₃)₆) precursor can beused as the aluminum source for the deposited film. By way of exampleand not limitation, the formation of aluminum nitride can be describedas follows. Initially, first spacers 500, MTJ structures 300, and ILDlayer 245 are thermally pre-treated with ammonia (NH₃); for example, thewafer with MTJ structures 300 and interconnect layers 205 is exposed toammonia (NH₃) gas. Alternatively, first spacers 500, MTJ structures 300,and ILD layer 245 can be treated with a plasma. In some embodiments, theplasma can include (i) a mixture of ammonia with helium, hydrogen, orargon; (ii) a mixture of nitrogen with helium, hydrogen, or argon. Theplasma treatment can be performed at a temperature similar to thedeposition temperature (e.g., between about 150° C. and about 400° C.)and at a process pressure similar to the deposition process pressure(e.g., between about 0.5 Torr and about 10 Torr). As a result of thepre-treatment (e.g., thermal or plasma), amino radicals (NH₂) can bechemisorbed on the exposed surfaces of first spacers 500, MTJ structures300, and ILD layer 245. Unreacted NH₃ gas can be subsequently removedwith a purge. A TMA soak can follow, where the precursor is introducedand allowed to chemically react with the amino radicals on the exposedsurfaces. As a result of the reaction, the precursor partiallydecomposes (e.g., releases a methyl group) and attaches itself to theamino radicals on the surface. A subsequent purge removes any unreactedTMA precursor along with the byproducts (e.g., methane, CH₄) of thereaction between the amino radicals and the TMA precursor. NH₃ gas isthen re-introduced to react with the partially decomposed precursor toform aluminum nitride. A subsequent purge removes the byproducts of thechemical reaction (e.g., methane) and any unreacted NH₃ gas.

The above process sequence produces aluminum nitride films with athickness that ranges from about 2 Å to about 5 Å depending on theprocess conditions (e.g., process pressure, temperature, gas andprecursor flow, etc.). Consequently, the deposition process sequence canbe repeated as needed until the desired thickness of aluminum nitridemetallic compound layer is achieved (e.g., between about SA and about 50Å).

In some embodiments, a post-treatment can be used to densify thedeposited aluminum nitride; e.g., by removing hydrogen. For example, aradio frequency (RF) NH₃ plasma can be used to remove hydrogen from thealuminum nitride layer. The RF power applied to the plasma can rangefrom about 100 Watts to about 500 Watts, according to some embodiments.However, the aforementioned RF power range should not be consideredlimiting and other ranges may be used. By way of example and notlimitation, the post-treatment can last for up to one minute and can beperformed in-situ with the deposition process described above. Thenitrogen to aluminum ratio (N/Al) of the resulting aluminum nitridelayer can be between about 0.67 and about 2.3. Consequently, thealuminum atomic percentage in the aluminum nitride layer can range fromabout 40 to about 70, and the nitrogen atomic percentage from about 30to about 60, as measured by X-ray photoelectron spectroscopy (XPS) orother appropriate method. Since the source of aluminum is TMA (e.g., anorganometallic precursor), traces of residual carbon can be found in thealuminum nitride layer. For example, the carbon atomic percentage in thealuminum nitride layers can range from about 1 to about 2. In someembodiments, the dielectric constant of the treated aluminum nitride canbe between about 7 and about 8, and its density between about 2 g/cm³and about 3 g/cm³, as measured by X-ray reflectometry. Due to their highdielectric constant and density, thin aluminum nitride layers (e.g.,less than 50 Å) can provide improved electrical isolation compared tolayers with lower dielectric constant such as silicon oxide, siliconnitride, silicon carbon nitride, and the like.

The deposition of aluminum oxide metallic compound layers is verysimilar to the process sequence described above for aluminum nitride.However, in the case of aluminum oxide deposition, the NH₃ exposureafter the TMA purge can be replaced with 1-butanol (C₄H₉OH) soak, wherethe partially decomposed TMA precursor can react with 1-butanol to formaluminum-oxygen bonds, and ultimately aluminum oxide. Similarly to thecase of aluminum nitride, a post-treatment with RF NH₃ plasma can beused to densify the as-deposited aluminum oxide metallic compound layerby removing hydrogen. The RF power applied to the plasma can range fromabout 100 Watts to about 500 Watts, according to some embodiments. Theresulting aluminum nitride film can have an aluminum to oxygen ratio(A/O) that ranges from about 1.2 to about 2.3. Further, the aluminumatomic percentage can be between about 30 to about 45, while thenitrogen atomic percentage can be between about 55 to about 70, asmeasured by XPS. By way of example and not limitation, the hydrogenatomic percentage in the plasma treated aluminum oxide film can rangefrom 0 to about 3, as measured by Rutherford Backscattering Spectrometry(RBS). In some embodiments, the formed aluminum oxide metallic compoundlayer can have a dielectric constant between about 7 and about 9.5, andits density can be between about 3 g/cm³ and about 3.5 g/cm³, asmeasured by X-ray reflectometry. Due to their high dielectric constantand density, thin aluminum oxide layers (e.g., less than 50 Å) canprovide improved electrical isolation compared to layers with lowerdielectric constants such as silicon oxide, silicon nitride, siliconcarbon nitride, and the like.

In referring to FIGS. 1 and 7 , method 100 continues with operation 140,where an etchback process can be used to recess metallic compound layer600 so that a second spacer 600 can be formed over each sidewall of MTJstructures 300. In some embodiments, the etchback process will partiallyremove metallic compound layer 600 over the top surface of ILD layer 245and will expose portions of top electrode 220 of MTJ structures 300, asshown in FIG. 7 . In some embodiments, the etchback process of operation120 can be used over in operation 140. For example, the etchback processof operation 140 can use an ion beam etch process or a reactive ionetching (RIE) process with an inductively coupled plasma (RIE-ICP). Insome embodiments, the ion beam etch process can use an ion beam thatselectively targets areas where the etching will be performed. The ionbeam can be produced from a helium (He), neon (Ne), argon (Ar), krypton(Kr), or xenon (Xe) plasma. By way of example and not limitation, theenergy of the ion beam during the etch process can be between about 100eV and about 1200 eV. Alternatively, the RIE-ICP process can useetchants such as tetrafluoromethane (CF₄), fluoroform (CH₂F₂), chlorine(Cl₂), Ar, He, organic gases, or combinations thereof. The etchbackprocess used to form the first and second spacers (e.g., first spacer500 and second spacer 600) is not limiting to the etchback processesdescribed above. Therefore, alternative etchback processes can be usedto form first spacer 500 and second spacer 600. Further, in someembodiments, the aluminum nitride or aluminum oxide metallic compoundlayers 600 can exhibit an etch selectivity from about 2:1 to about 10:1(e.g., 2:1, 5:1, 8:1, 10:1). However, the aforementioned selectivityrange is not limiting and higher selectivity ratios are possible (e.g.,50:1).

In referring to FIG. 1 , method 100 continues with operation 150 and theformation of a third spacer over the second spacer (e.g., etchedmetallic compound layer 600). Referring to FIG. 8 , a third spacer 800can be blanket-deposited over metallic compound layer 600. In someembodiments, third spacer 800 includes a dielectric material, such assilicon nitride, silicon carbon nitride, silicon oxide, carbon dopedsilicon oxide, or silicon oxy-carbon nitride with a thickness betweenabout 100 Å and about 500 Å (e.g., about 150 Å). Therefore, in someembodiments, third spacer 800 can be thicker than metallic compoundlayer 600 (e.g., second spacer). As discussed above, pitch P between MTJstructures 300, shown in FIG. 1 , can range from about 92 nm and 80 nm.Therefore, the spacing between MTJ structures 300, after the formationof the first and second spacers, will be reduced to less than 80 nm.Consequently, it is desirable that third spacer 800 can be depositedwith a deposition method with enhanced gap-fill characteristics. By wayof example and not limitation, third spacer 800 can be deposited byPEALD, CVD, ALD, or a deposition method with enhanced gap-fillcapabilities so that third spacer 800 can fill the space between MTJstructures 300, as shown in FIG. 8 .

According to some embodiments, first spacer 500, metallic compound layer600 (second spacer), and third spacer 800 form a spacer stack wheremetallic compound layer 600 (second spacer) is thinner than first spacer500 and third spacer 800. Further, the spacer stack can have a thicknessthat ranges from about 30 nm.

In some embodiments, a dielectric layer 805 can be deposited over thirdspacer 800. By way of example and not limitation, dielectric layer 805can be a silicon oxide (SiO₂) interlayer dielectric grown withtetraethoxysilane (TEOS). Alternatively, dielectric layer 805 can be alow dielectric constant interlayer dielectric; e.g., with a dielectricconstant lower than 3.9. In some embodiments, dielectric layer 805 canhave a thickness of about 500 Å or more. A chemical mechanicalplanarization (CMP) process can polish dielectric layer 805 so that thetop surface of top electrodes 220 and the top surface of dielectriclayer 805 can be substantially coplanar, as shown in FIG. 9 . Accordingto some embodiments, additional interconnect layers (not shown in FIG. 9) can be formed over the planarized surfaces of dielectric layer 805 andMTJ structures 300 so that top electrodes 220 can be connected torespective vias in the interconnect layer over MTJ structures 300.

In some embodiments, spacer stacks with metallic compound layers, suchas aluminum nitride or aluminum oxide, can exhibit improved dielectricproperties compared to spacer stacks limited to silicon-based layerssuch as silicon nitride, silicon carbon nitride, silicon oxide, carbondoped silicon oxide, or silicon oxy-carbon nitride. Therefore, spacerstacks with metallic compound layers can be compact (e.g., with athickness below about 50 Å) and suitable for MRAM cells with a tightpitch between MTJ structures (e.g., between about 80 nm and about 92nm). Further, spacer stacks with metallic compound layers, exhibitimproved etch characteristics. For example, spacer stacks with metalliccompound layers can demonstrate improved etch resistance and selectivity(e.g., between about 2:1 and about 10:1). As a result, spacer stackswith metallic compound layers can be less susceptible to etch damageduring a subsequent etchback process.

The present disclosure is directed to an exemplary fabrication methodfor forming spacer stacks with metallic compound layers therein. In someembodiments, the metallic layers can improve the electrical isolationbetween adjacent MTJ structures. By way of example and not limitation,the spacer stack can include a 5 nm, or thinner (e.g., about 3 nm),metallic compound layer that can include aluminum oxide, aluminumnitride, titanium oxide, titanium nitride, ruthenium oxide, or any othersuitable material. The metallic compound layer can be interposed betweenlayers of silicon nitride, silicon carbon nitride, silicon oxy-nitride,silicon oxy-carbon nitride, silicon oxide, or combinations thereof toform a spacer stack on each sidewall surface of the MTJ structure. Insome embodiments, the metallic compound layer can be conformallydeposited with a thermal atomic layer deposition or a plasma-enhancedatomic layer deposition process. According to some embodiments, spacerstacks with metallic compound layers exhibit improved electricalisolation characteristics compared to spacer stacks without metalliccompound layers and can therefore be more compact (e.g., thinner).

In some embodiments, a method includes forming magnetic tunnel junction(MTJ) structures on an interconnect layer. The method also includesdepositing a first spacer layer over the MTJ structures and theinterconnect layer, where the first spacer layer is etched to expose thetop electrodes of the MTJ structures and the interconnect layer. Themethod further includes depositing a second spacer layer over the firstspacer material, the MTJ structures, and the interconnect layer; wherethe second spacer layer is thinner than the first spacer layer andincludes a metallic compound. Additionally the method includes etchingthe second spacer layer to expose the top electrodes of the MTJstructures.

In some embodiments, a structure includes a substrate with transistors,contact layers, and one or more interconnect layers disposed thereon.The structure further includes one or more MTJ structures over the oneor more interconnect layers, where bottom electrodes of the one or moreMTJ structures are disposed on vias in a top interconnect layer of theone or more interconnect layers. The structure also includes a firstspacer on each sidewall surface of the one or more MTJ structures and ametallic compound layer over each sidewall surface of the one or moreMTJ structures and over the top interconnect layer, where the metalliccompound layer forms a second spacer that is thinner than the firstspacer. The structure also includes a third spacer on the metallicinterconnect layer, where the third spacer is thicker than the secondspacer and is disposed between the MTJ structures.

In some embodiments, a structure includes an interconnect layer withlines and vias over a substrate and MTJ structures disposed over thevias in the interconnect layer, where the bottom electrodes of the MTJstructures are in contact with vias in the interconnect layer. Thestructure further includes a first spacer on each sidewall surface ofthe MTJ structures and a second spacer over the first spacer and theinterconnect layer, where the second spacer includes a metallic compoundlayer.

The foregoing outlines features of embodiments so that those skilled inthe art may better understand the aspects of the present disclosure.Those skilled in the art should appreciate that they may readily use thepresent disclosure as a basis for designing or modifying other processesand structures for carrying out the same purposes and/or achieving thesame advantages of the embodiments introduced herein. Those skilled inthe art should also realize that such equivalent constructions do notdepart from the spirit and scope of the present disclosure, and thatthey may make various changes, substitutions, and alterations hereinwithout departing from the spirit and scope of the present disclosure.

What is claimed is:
 1. A method, comprising: forming magnetic tunneljunction (MTJ) structures on an interconnect layer, wherein forming theMTJ structures comprises recessing top surfaces of the interconnectlayer with respect to bottom surfaces of the MTJ structures; depositinga first spacer layer on the MTJ structures and the interconnect layer,wherein the first spacer layer is etched to expose top electrodes of theMTJ structures and the interconnect layer, and wherein depositing thefirst spacer layer comprises forming a bottom surface of the firstspacer layer below an interface between the interconnect layer andbottom electrodes of the MTJ structures; depositing a second spacerlayer, comprising a metal, on the first spacer layer, the MTJstructures, and the interconnect layer, wherein the second spacer layeris thinner than the first spacer layer; performing a plasma process onthe second spacer layer to remove hydrogen atoms from the second spacerlayer and densify the second spacer layer; and etching the second spacerlayer to form a continuous metal-based layer directly on portions of thefirst spacer layer facing each other and directly on a portion of adielectric between the MTJ structures and to expose the top electrodesof the MTJ structures.
 2. The method of claim 1, wherein depositing thesecond spacer layer comprises: exposing the first spacer layer, the MTJstructures, and the interconnect layer to an ammonia plasma or anitrogen plasma; exposing the first spacer layer, the MTJ structures,and the interconnect layer to a trimethylaluminum precursor to form aprecursor layer; and exposing the precursor layer to an ammonia gas orto a 1-butanol gas to form an aluminum nitride or an aluminum oxide,respectively.
 3. The method of claim 1, wherein depositing the secondspacer layer comprises depositing a metallic compound comprisingaluminum oxide with an aluminum to oxygen ratio between about 1.2 andabout 2.3 and a dielectric constant between about 7 and about 9.5. 4.The method of claim 1, wherein depositing the second spacer layercomprises depositing a metallic compound comprising aluminum nitridewith an aluminum to nitrogen ratio between about 0.67 and about 2.3 anda dielectric constant between about 7 and about 9.5.
 5. The method ofclaim 1, wherein depositing the second spacer layer comprises depositinga metallic compound comprising titanium nitride, titanium oxide, orruthenium oxide.
 6. The method of claim 1, wherein depositing the firstspacer layer comprises depositing silicon nitride, silicon carbonnitride, or combinations thereof.
 7. The method of claim 1, whereindepositing the second spacer layer comprises forming a bottom surface ofthe second spacer layer below the bottom surfaces of the MTJ structures.8. The method of claim 1, wherein depositing the first spacer layercomprises forming a bottom surface of the first spacer layer below a topsurface of a conductive structure in the interconnect layer.
 9. Amethod, comprising: forming an interconnect layer on a substrate,wherein the interconnect layer comprises conductive structurescomprising barrier layers and metal layers surrounded by a dielectric;forming magnetic tunnel junction (MTJ) structures on the conductivestructures of the interconnect layer, wherein forming the MTJ structurescomprises recessing top surfaces of the dielectric with respect tobottom surfaces of the MTJ structures and top surfaces of the conductivestructures; depositing a first spacer layer comprising a dielectricmaterial directly on the MTJ structures and the interconnect layer;etching the first spacer layer to form first spacers on sidewallsurfaces of the MTJ structures so that bottom surfaces of the firstspacers are below the bottom surfaces of the MTJ structures and the topsurfaces of the conductive structures; depositing a second spacer layercomprising a metal directly on the first spacers, the MTJ structures,and the interconnect layer; and etching the second spacer layer to forma second spacer as a continuous layer directly on two of the firstspacers facing each other and directly on a portion of the dielectricbetween the two of the first spacers, wherein a bottom surface of thesecond spacer is below top surfaces of the barrier layers and the metallayers of the conductive structures.
 10. The method of claim 9, whereinetching the second spacer layer comprises forming the second spacerthinner than the first spacers.
 11. The method of claim 9, whereinetching the first and second spacer layers comprises forming the firstspacers and the second spacer with coplanar bottom surfaces.
 12. Themethod of claim 9, wherein etching the second spacer layer comprisesforming the first spacer and the second spacer so that each of the firstspacers and the second spacer is in contact with a different sidewallportion of a top electrode in the MTJ structures.
 13. The method ofclaim 9, wherein etching the second spacer layer comprises forming thesecond spacer on an entire top surface of the dielectric.
 14. The methodof claim 9, wherein depositing the second spacer layer comprises:flowing a trimethylaluminum precursor to form a decomposed precursorlayer; and exposing the decomposed precursor layer to an ammonia gas orto a 1-butanol gas to form an aluminum nitride or an aluminum oxide,respectively.
 15. A method, comprising: forming first and secondmagnetic tunnel junction (MTJ) structures on conductive structures of aninterconnect layer; depositing a first spacer layer comprising adielectric material on the first and second MTJ structures and theinterconnect layer; etching the first spacer layer to form first spacerson sidewall surfaces of the first and second MTJ structures so thatbottom surfaces of the first spacers are not coplanar with bottomsurfaces of the first and second MTJ structures; depositing a secondspacer layer comprising a metal on the first spacers, the first andsecond MTJ structures, and the interconnect layer; etching the secondspacer layer to form a second spacer as a continuous layer directly onthe first spacers and between the first and second MTJ structures and toexpose sidewalls and top surfaces of top electrodes of the first andsecond MTJ structures; and forming a third spacer comprising thedielectric material directly on the second spacer and directly onsidewalls of the top electrodes of the first and second MTJ structures,wherein each of the first spacers, the second spacer, and the thirdspacer are in direct contact with different sidewall portions of the topelectrodes of the first and second MTJ structures.
 16. The method ofclaim 15, wherein etching the second spacer layer comprises forming abottom surface of the second spacer coplanar with the bottom surfaces ofthe first spacers.
 17. The method of claim 15, wherein etching thesecond spacer layer comprises forming a bottom surface of the secondspacer not coplanar with the bottom surfaces of the MTJ structures. 18.The method of claim 15, wherein etching the second spacer layercomprises forming a bottom surface of the second spacer below a topsurface of the conductive structures.
 19. The method of claim 15,wherein depositing the second spa cer layer comprises depositing a metaloxide.
 20. The method of claim 15, wherein depositing the second spacerlayer comprises depositing a metal oxide comprising a metal nitride.